1. Field of the Invention
This invention relates generally to a method and system for controlling aeromechanical instabilities (flutter) in rotary compression systems such as aircraft gas turbine engines. More particularly, this invention relates to sensing rotary blade characteristics of a rotary compressor or the flow asymmetry produced by blade movement to minimize flutter instability conditions.
2. Brief Description of the Art
Flutter is aeromechanical instability that is experienced near the stall line of a performance map due to blade motion.
Flutter imposes constraints on the performance of rotary compressors, such as gas turbine engines. Flutter is caused by blade motion or deflection and can be viewed as a two-dimensional phenomena that results in a region of reduced or reversed fluid flow through the compressor causing the compressor to reduce output. Flutter instability can degrade the performance of the rotary compressor and may also lead to fatigue failure or other permanent damage to the compressor. One result of the flutter instability can be blade deformation and/or blade fatigue failure. Thus, it is desirable to avoid rotary compressor blade motion that causes flutter.
One possible solution to reduce the effects of flutter in a rotary compressor is to lower the operating line of the compressor by shutting down the compressor and restarting it. Unfortunately, this results in substantial performance penalties for the compressor.
Thus, what is needed to solve flutter instability, encountered by rotary compressors, is a technique to optimize performance while avoiding flutter disturbances. A solution to eliminating stall and/or surge is disclosed in WO Patent Application Serial No. 9700381, with a priority date of Nov. 2, 1995 entitled, xe2x80x9cCompressor Stall and Surge Control Using Airflow Asymmetry Measurementxe2x80x9d, which is hereby incorporated herein by reference in its entirety. The stall and/or surge approach in the above-cited patent application does not solve the problem of flutter instability. Flutter is distinguished from rotating stall and surge because rotating stall and surge occurs without mechanical motion, while flutter is a function of blade motion. The blade movement, and associated deformation or deflection of the blade is the source of flutter instability. Stall and surge are aerodynamic instabilities resulting from a compressor operating in excess of its rated capacity.
Another example of the control of unsteady motion phenomena may be found in U.S. Pat. No. 4,967,550 entitled xe2x80x9cActive Control of Unsteady Motion Phenomena in Turbomachineryxe2x80x9d which is hereby incorporated herein by reference in its entirety. The aforementioned U.S. Patent describes a control system for actively controlling at least one mode of unsteady motion phenomena in turbomachinery in order to increase the operating range of the turbomachinery.
One advantage of the present invention is to provide a control system that facilitates operation of a rotary compressor at an optimal operating mode, while avoiding the flutter instability characteristics.
Accordingly, one aspect of the instant invention is drawn to a system for reducing flutter instabilities in a rotary compressor having a plurality of blades that comprises a system for reducing flutter characteristics in a rotary compressor having a plurality of blades comprising:
a plurality of sensors for sensing vibrations resulting from deformation movement of the blade and generating a flutter signal that is a function of the vibrations;
a signal conditioning circuit, coupled to each of the sensors for receiving the flutter signals and processing the flutter signals to produce a composite signal that is a function of the flutter signals;
a computation circuit, coupled to the signal conditioning circuit, for receiving the composite signal and generating an amplitude signal that is a function of the composite signal;
a flutter control circuit, coupled to the computation circuit, for receiving the amplitude signal and generating a control signal that is a function of the amplitude signal;
an actuator, coupled to the flutter control circuit, for receiving the control signal and responding to the control signal by modulating an annulus averaged flow through the compressor thereby reducing flutter characteristics on the plurality of blades.
A second aspect of the instant invention is a process for reducing flutter in a rotary compressor system that comprises a method for reducing flutter characteristics in a rotary compression system comprising:
sensing vibration produced by a rotating blade;
generating a flutter signal that is a function of the sensed vibration;
transmitting the flutter signal to a processor;
generating a control signal based on the flutter signal; and transmitting the control signal to an actuator for controlling the position of the actuator, thereby modulating an annulus averaged flow through the compressor.
A third aspect of the instant invention is drawn to a method for reducing flutter instability of a rotary compressor wherein the steps of the method are stored on a computer-readable medium and comprise a method for reducing instability of a rotary compressor stored on a computer-readable medium comprising:
generating a substantially parabolic flutter boundary curve representing flutter parameters of the rotary compressor;
operating the rotary compressor in a substantially linear mode of operation that is in accordance with substantially optimum operating parameters of the rotary compressor;
sensing flutter vibrations of the compressor;
calculating a differential quantity representative of the difference between the flutter boundary curve and the operating mode;
comparing the flutter vibrations to the differential quantity;
operating the rotary compressor in a substantially nonlinear mode of operation when the magnitude of the flutter vibration is greater than the differential quantity;
monitoring the relationship of the magnitude of the flutter vibration and the differential quantity; and operating the rotary compressor in the substantially linear mode of operation when the flutter vibration is less than the differential quantity.